actions: manifest: use a manifest specitic dnm label

Reconfigure the manifest action to use a manifest specific DNM label, so
that the "DNM" one can be used by humans.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

Author: fabiobaltieri

.github/workflows/manifest.yml

@@ -36,4 +36,4 @@ jobs:
           label-prefix: 'manifest-'
           verbosity-level: '1'
           labels: 'manifest'
-          dnm-labels: 'DNM'
+          dnm-labels: 'DNM (manifest)'